Onega stepanovi

Etymology

The generic name Onega comes from the Onega Peninsula of the White Sea, where the first fossils were found. The specific name is given in honour of V. A. Stepanov, discoverer of the first Ediacaran fossil locality in Arkhangelsk Region, on the Letniy Bereg (Summer Coast) of the Onega Peninsula in 1972.

Morphology

The small fossils, which range up to 7 millimetres (0.28 in) long, have oval outlines and low bodies with an articulated central zone built of isomers encircled by an undivided zone. The surface of the undivided region of Onega is covered with small tubercles.

Onega was originally described by Mikhail Fedonkin as a problematic organism, being grouped together with Vendia, Praecambridium and Vendomia as possible stem-group arthropods due to a vague similarity with primitive Cambrian trilobites and arthropods.

In 1985 Mikhail Fedonkin erected Phylum Proarticulata, in which he placed: Onega, Dickinsonia, Palaeoplatoda, Vendia, Vendomia, Praecambridium and Pseudovendia sp., although he did not exclude the possibility that Onega may still be related to various lower Cambrian arthropods, such as Skania.

Andrey Yu. Ivantsov has proposed that Onega be placed in phylum Proarticulata, as the segments in recently discovered, exceptionally well-preserved fossils display the glide, or "staggered", symmetry characteristic of the majority of proarticulatans.

Fossil record

Imprints of Onega stepanovi have been found in the Verkhovka and Zimnie Gory Formations of the Ediacaran (Vendian) rocks of the Arkhangelsk Region, Russia.

All the fossil specimens are negative imprints on the bases of fine-grained sandstone beds with the "elephant skin" and tubercle texture diagnostic of microbial mats. The same bedding planes contain various other Ediacaran species: Cyclomedusa, Ediacaria, palaeopascichnids, Eoporpita, Yorgia, Andiva, Archaeaspinus, Vendia, Dickinsonia, Anfesta, Albumares, Tribrachidium, Kimberella, Parvancorina, Charniodiscus and others.